1. Field of the Invention
The present invention relates to a vertical cavity surface-emitting semiconductor laser device, an optical transmission module, an optical transmission device, and an optical switching method.
2. Description of the Related Art
In recent years, the optical transmission technology has been developed to not only the trunk-system transmission network but also LAN, the access system and the home network. For example, in the Ethernet, the data-transmission capacity of 10 Gbps has been developed. In the future, a further increase in the data-transmission capacity is demanded, and an optical transmission system with the transmission capacity of over 10 Gbps is expected.
In the light source for optical transmission the data-transmission capacity of which is 10 Gbps or less, a direct modulation method which modulates the output light intensity by modulating the introducing current of a semiconductor laser is mainly used.
However, it is difficult to operate the existing semiconductor laser at a modulation frequency exceeding 10 GHz by using the direct modulation method.
To overcome the problem, the system which modulates the light outputted from the semiconductor laser by using an external modulator is developed for a light source for optical transmission with the transmission capacity of over 10 Gbps.
However, in the case of the external modulator system, the module size is large, the number of parts needed is also large, and there is a problem that the external modulator system is expensive. Therefore, even if the optical transmission technology employing the external modulator can be used for an expensive system such as the trunk system, it is unsuitable for the systems used by the ordinary users, such as LAN systems or home network systems.
A vertical cavity surface-emitting semiconductor laser device (VCSEL) is increasingly used as a light source for LAN or optical interconnection system. When compared with the conventional end-face emitting type semiconductor laser, this VCSEL has a low power consumption, and the cleavage at the manufacturing process is unnecessary. The inspection of the elements of the VCSEL in the wafer state is also possible, and the VCSEL is excellent in promoting low cost production.
Therefore, the VCSEL using the direct modulation is expected as a light source for large-capacity optical LAN and optical interconnection with the transmission capacity of over 10 Gbps.
Conventionally, some methods for modulating VCSEL at high speed are disclosed in Japanese Laid-Open Patent Application No.05-152674, Japanese Laid-Open Patent Application No.2003-202529, Japanese Laid-Open Patent Application No.05-063301, and Appl. Phys. Lett. vol. 76, pp. 1975-1977 (2000).
Namely, Japanese Laid-Open Patent Application No.05-152674 discloses a surface-emitting laser in which an optical absorption layer is provided in the middle of the upper multilayer film reflector of a VCSEL. This surface-emitting laser is provided with an external modulator which applies a reverse bias to the optical absorption layer to change an absorption coefficient of the external modulator. In the surface-emitting laser, a laser light beam is emitted by supplying a steady current to an active layer, and the rate of optical absorption of the external modulator is changed, so that the output light is turned on and off.
Japanese Laid-Open Patent Application No.2003-202529 discloses a laser device in which an optical absorption layer in the form of multiple quantum well structure is provided in a resonator of a VCSEL. A reverse bias is applied to the optical absorption layer, and an optical absorption coefficient is changed. Thereby, the internal absorption loss of the VCSEL is changed, and the oscillation threshold is changed. Even when a fixed current is supplied to the active layer, the laser light output intensity can be modulated greatly.
Appl. Phys. Lett. vol. 76, pp. 1975-1977 (2000), entitled “Q-switched Operation of a Coupled-Resonator Verical-Cavity Laser Diode”, discloses a coupled-resonator vertical-cavity surface-emitting laser device (VCSEL). In this VCSEL, an active layer is provided in one of the two resonators, and a GaAs absorption layer is provided in the other resonator. A direct current is supplied to the active layer and a reverse bias is applied to the GaAs absorption layer, so that the laser light output intensity is modulated at 2 GHz (optical switching).
Japanese Laid-Open Patent Application No.05-063301 discloses a laser device in which a refractive index of one of a pair of distribution Bragg reflectors (DBR) of a VCSEL is changed by electric field or carrier introducing, or optical introducing, and the resonance wavelength of the distribution Bragg reflectors is changed and the wavelength difference between the resonance wavelength of the distribution Bragg reflectors and the optical emission wavelength is changed, so that laser light output intensity is modulated at high speed.
In the laser device of Japanese Laid-Open Patent Application No.05-152674, the multilayer film reflectors are provided on the upper and lower sides of the optical absorption layer, and the rate of the absorption of light is increased according to the resonance effect, and the modulation is performed. If the wavelength of the light is in agreement with the resonance wavelength, the absorptivity of the light is changed greatly. However, since the thickness of the optical absorption layer is small, if the wavelength of the light is different from the resonance wavelength, a change of the absorptivity of the light is small. Therefore, in order to operate the above-mentioned laser device effectively, it is necessary that the oscillation wavelength of the VCSEL strictly coincides with the resonance wavelength of the optical absorption layer. Thus, the production of the laser device is difficult.
In each of the laser devices of Japanese Laid-Open Patent Application No.2003-202529, Japanese Laid-Open Patent Application No.05-063301, and Appl. Phys. Lett. vol. 76, pp. 1975-1977 (2000), the laser light output intensity is modulated by changing the absorption loss or the refractive index of the resonator while the fixed current is introduced into the active layer. In this case, the carrier density inside the active layer is changed by a change of the optical output power inside the laser device. A certain time is needed for accumulating the carrier in the active layer, and this is the cause of a decrease of the modulation speed.